Declare nullable params explicitly null.

src/Factory.php

@@ -63,7 +63,7 @@ class Factory
      * @return T
      * @throws ClassDoesNotExistsException
      */
-    public static function create($class, Authentication $authentication = null)
+    public static function create($class, ?Authentication $authentication = null)
     {
         if (class_exists($class)) {
             return new $class($authentication);

src/Request/Customer.php

@@ -267,7 +267,7 @@ class Customer extends AbstractSerializer
      *
      * @param Address|null $billingAddress Billing address
      */
-    public function __construct(Address $billingAddress = null)
+    public function __construct(?Address $billingAddress = null)
     {
         $this->billing = $billingAddress;
     }

src/Response/AbstractResponse.php

@@ -49,7 +49,7 @@ abstract class AbstractResponse
      * @return void
      * @throws \InvalidArgumentException
      */
-    public function headerSetter(\SimpleXMLElement $xml = null)
+    public function headerSetter(?\SimpleXMLElement $xml = null)
     {
         if ($xml) {
             $this->Header = ResponseSerializer::serialize(Header::class, $xml);
@@ -64,7 +64,7 @@ public function headerSetter(\SimpleXMLElement $xml = null)
      * @return static
      * @throws \InvalidArgumentException
      */
-    public function deserialize(\SimpleXMLElement $xml = null)
+    public function deserialize(?\SimpleXMLElement $xml = null)
     {
         $object = clone $this;
 

src/Serializer/ResponseSerializer.php

@@ -45,7 +45,7 @@ class ResponseSerializer
     public static function serialize(
         $objectName,
         \SimpleXMLElement $data,
-        \SimpleXMLElement $header = null
+        ?\SimpleXMLElement $header = null
     ) {
         $object = new $objectName();
         $object->headerSetter($header);